Many rotary positive displacement devices have been devised and a common characteristic of positive displacement devices compared to centrifugal devices is the components of a positive displacement device come in contact during operation or have a thin fluid boundary. Contact can occur during start up and start up procedures as well as during certain operational environments. Some of the advantages of the present invention are discussed herein.
The invention has a wear-in tendency of the contact surfaces. The length of the lobe tips for the first and second rotor sets are staggered. The longer lobes seal only at the top of the rotation, and the shorter lobes seal only at the bottom of the rotation. The relative movement of the contacting surfaces at these points is such that a contact seal is maintained even if wear occurs on the rotors. Further, the lobes have a tendency to be self seating. Because the rotors are self-seating, there is less of a need for precision machining in many applications. Once seated, the rotors will provide that same seal integrity until the entire seal surface has been worn away allowing lower production cost. Other advantages of low precision manufacturing, allow less expensive fabrication of the rotors for various applications.
The lobes further are self sealing. The non driven rotor does not float rotationally but rather is held with a consistent “half force” against the driven rotor by the pressure of the fluid. A fluid film will still separate the two rotors at certain speeds and pressures with certain fluids; however, if this does not occur there is little detriment because the sliding velocity in the seal portion of the rotation is very low (reducing wear).
The design further has the tendency to create a device with increased efficiency. The present invention has slightly greater internal turbulence then the other rotary positive displacement pumps and motors; however, the present invention has greater volumetric efficiency.
The ability of the non-driven rotor to rotate slightly to make up for wear also allows it to rotate the other direction to allow larger chunks to pass through. In one example, rotors could allow 0.040″ particles to pass through without damaging the rotors. Higher clearances are also possible. The variable gap for chunk pumping is ideal for many applications from mine pumps, coal pumps to jet-skis, etc.
In one form the present invention allows for replaceable seals. As seen in an embodiment herein there is only a small area that seals on each lobe allowing a simple manufacturing of the rotors from an easily fabricated material and then attach some wear tips thereto. Wear tips could be made from more expensive, resilient material while the remainder of the rotor assembly is constructed from less expensive material.